1. Field of the Invention
The present invention relates to an X-ray image intensifier and a method of manufacturing the same and, more particularly, to an improvement of an input phosphor screen of the X-ray image intensifier.
2. Description of the Related Art
A system for observing an object to be imaged by using an X-ray image intensifier generally has an arrangement shown in FIG. 1. An X-ray image intensifier 2 is placed in front of an X-ray source 1. A X-ray beam which becomes modulated as it is transmitted through an object 3 to be imaged is incident on the X-ray image intensifier 2. An output image obtained in the X-ray image intensifier 2 is observed through an imaging camera and can be reproduced on a monitor TV.
In this case, an input screen 4 is arranged at one end of the X-ray image intensifier 2, and an output phosphor screen 5 is arranged at the other end of the image intensifier 2 so as to oppose the input screen 4. During an operation of the system, a modulated X-ray image is converted into an optical image by the input screen 4. This optical image is then converted into a photoelectronic image. When the photoelectronic image is focused and accelerated, a luminance-intensified output image is obtained on the output phosphor screen 5. This output image is observed through, e.g., an imaging camera.
The input screen 4 of the conventional X-ray image intensifier 2 has an arrangement shown in FIG. 2. A phosphor layer 8 constituted by columnar crystals 7 consisting of a CsI:Na phosphor is formed on the concave surface of a spherical aluminum substrate 6. The input phosphor screen is constituted by the aluminum substrate 6 and the phosphor layer 8. A photoelectric screen 10 is formed on the phosphor layer 8 of the input phosphor screen through an intermediate layer 9 consisting essentially of aluminum oxide and indium oxide layers.
In order to reduce exposure of the object 3 to X-rays, X-rays which are transmitted through the object must be input in the phosphor layer 8 without a loss to increase an absorption amount of the X-rays. With regard to the phosphor layer 8, in order to increase the X-ray absorption amount, the phosphor columnar crystals 7 are preferably elongated. However, if the columnar crystals 7 are elongated, the length of light propagation from a side surface of a given columnar crystal 7 to another columnar crystal 7 is increased, resulting in a decrease in resolution. For this reason, the columnar crystals 7 cannot be elongated much, and the maximum length of each columnar crystal is about 400 .mu.m.
Attempts to solve the above-described problem have been made. For example, Published Examined Japanese Utility Model Application No. 48-2465 discloses a phosphor screen manufactured by forming a light-reflecting layer on the inner wall of each through hole of a fiber plate formed by laterally stacking a large number of tubular fibers, and embedding a fluorescent material in each through hole.
In this case, light emitted when the fluorescent material of each fiber absorbs X-rays is not transmitted through another adjacent fiber, but can reach the surface while being confined in the fiber. Therefore, if the diameter of each fiber is sufficiently decreased, a high-resolution phosphor screen is theoretically obtainable.
Intensifying screens used for X-ray diagnosis, however, currently have a maximum screen size of 14 inches. The view field diameter of the input screen of each X-ray image intensifier is six inches or more, and reaches a maximum of 22 inches. If such a large-diameter input screen is manufactured by the method disclosed in Published Examined Japanese Utility Model Application No. 48-2465, the manufacturing cost becomes prohibitive. Hence, such a method cannot be practically used.
If a commercially available fiber plate is used, and its core is removed by chemical etching, a plate without a core can be easily formed. After light-reflecting coating layers are formed on the inner walls of small holes in the fiber plate whose core is removed, the holes are filled with a phosphor, thereby obtaining an input phosphor screen with a high resolution.
In order to manufacture a fiber plate having a diameter of six inches or more, an enormous cost is required, and the manufactured plate would have insufficient heat resistance. Therefore, such a plate cannot be applied to the input phosphor screen of an X-ray image intensifier.
In addition, Japanese Patent Disclosure (KOKAI) No. 51-127668 discloses an input phosphor screen that is obtained by forming a large number of small holes in a metal substrate by chemical etching and filling the small holes with a phosphor, and the obtained input phosphor screen is used as the input screen of an X-ray image intensifier.
If, however, small holes are to be formed in a metal substrate by chemical etching, it is very difficult to set the ratio of the maximum inner diameter to the depth of each small hole to be one or less by using any available technique. For example, if the depth of each small hole is set to be 400 .mu.m in accordance with a thickness of 400 .mu.m (of a substrate) which is required when a fluorescent material to be filled in small holes is a phosphor containing CsI as a major component, the sectional size of each small hole can only be reduced to about 400 .mu.m.
An input phosphor screen, therefore, obtained by forming a large number of small holes each having a diameter of 400 .mu.m and a depth of 400 .mu.m in a metal substrate, and filling the small holes with a CsI phosphor has a limit resolution of about 20 lp/cm. In comparison with a limit resolution of 50 to 100 lp/cm of an existing 400 .mu.m thick CsI input phosphor screen, the resolution characteristics of the above-described input phosphor screen are expected to be greatly degraded.
In an RCA Review, "An X-Ray Sensitive Fiber Optic Intensifier Screen for Topography" is described by R. W. Smith. This article describes a phosphor screen obtained by removing the core portion of a fiber plate by etching to form small holes, and filling the small holes with a melted CsI:Na phosphor.
In order to apply this phosphor screen to an X-ray image intensifier for medical diagnosis, a fiber plate having a diameter of 6 inches or more is required. However, such a fiber plate is very expensive and hence is not suitable for practical applications. In addition, since a fiber plate has a low melting point, if a phosphor is melted and filled, the depth of each small hole is undesirably limited.